The present invention relates to a valve timing control system for controlling both the intake valve (opening/closing) time and the exhaust valve (opening/closing) timing.
A conventional valve timing control system is arranged to vary the rotational phase of a camshaft relative to a sprocket driven by an engine crankshaft, by the use of hydraulic pressure. Recently, there is provided a valve timing control system arranged to control the valve timings of the intake and exhaust valves by varying the rotational phase of the camshaft relative to the sprocket by transmitting a rotational force of an electric motor through a speed reducer to the camshaft.
In the valve timing control system disclosed in JP 2006-207398A (≈US2009048758A1), there is provided, for each of intake camshaft and exhaust camshaft, a valve timing control mechanism or apparatus driven by an electric motor.